It's possible that the seal's just worn out. Maybe the bike was sitting around a while. This can dry the seal out, which then goes brittle and starts to spew juice out.

Another possiblity is that the gearbox has been overhauled, and when it was refitted, the countershaft caught on the seal tensioning spring and pulled it off, or worse, tore the lip of the seal.

Either way, you can change this seal without removing even the gear oil.

Very carefully drill a couple of small holes 180 degrees apart in the seal. Thread a couple of sekf tapping screws in just deep enough to get purchase. Grab the screws with some meaty pliers and yank that puppy out.

Fitting the new seal is a simple task, just be careful sliding the seal onto the countershaft so as not to damage it. Then tap it home making sure it goes in squarely.

Job done.